Abstract: Fine root decomposition often plays an important role in maintaining soil fertility and mass nutrient cycling in soil ecosystems. As irreplaceable decomposers of fine roots, soil microorganisms are closely linked to bioelement components. Birch (Betula luminifera)-grass (Hemarthria compressa) is a successful forest and grass compound model for converting farmland to forest and is beneficial to the development of the environment and economy. There is a lack of information on quantities of soil microbes in relation to fine root decomposition and few published reports on soil microorganisms in birch-grass plantations. Therefore, soil microbial amount and dominant microbial groups during decomposition of fine roots and grass roots were investigated in order to obtain an understanding of the mechanisms of fine root decomposition. Birch fine roots and grass roots from a birch-grass compound model in converting farmland to forest in Hongya, Sichuan, were used in a 120-d laboratory incubation experiment. The roots were mixed with the same soil in the following five treatments: 1) 0-1 mm birch fine roots, 2) 1-2 mm birch fine roots, 3) grass roots, 4) 0-2 mm birch fine roots+grass roots, and 5) no roots (CK). Treatments of addition of fine roots (and grass roots) significantly increased soil microbial content. The ranked orders of total microbial content were: Treatment 4＞treatment 3＝treatment 1＞treatment 2 (P＜0.05). Fungi played a more important role in fine root decomposition than bacteria and actinomycetes. Eight dominant microbial groups were isolated and identified. Pseudomonas, Trichoderma, and Actinoplanes played the most important role at the initial phase of fine and grass root decomposition, while Bacillus, Aspergillus, Penicillium, Streptomyces, and Nocardia dominated the later phase of decomposition. Regular fluctuations were not observed during the first 120 d of the decomposition processes but after 120 d eight dominant microbial groups increased significantly (P＜0.05). The different treatments with birch fine roots and grass roots greatly influenced soil microbial amount and the dominant microbial groups in root decomposition.